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It is said that pessimists are never disappointed, but a new study suggests they are also less stressed.

Scientists at University College London have discovered that it is uncertainty that leaves people feeling anxious and unable to cope, while accepting the inevitable brings a sense of calm.

It explains why fear of the dentist is often worse than actually attending a check-up or why the families of missing persons feel better knowing what has happened to loved ones, even if it turns out to be bad news.

“The most stressful scenario is when you really don’t know,” said Dr Robb Rutledge of UCL’s Institute of Neurology.

“When applying for a job, you’ll probably feel more relaxed if you think it’s a long shot or if you’re confident that it’s in the bag.

“The same is likely to apply in many familiar situations, whether it’s waiting for medical results or information on train delays.”

To test how much stress was based on feelings of uncertainty, the researchers asked 45 students to play a computer game where they turned over rocks to find out if they had snakes underneath. If they found a snake they received a mild electric shock.

Volunteers were given a mild electric shock if they found a snake under a rockCredit:
Dirk Rueter / Alamy Stock Photo

Over time, the participants learned which rocks were more likely to harbour snakes and sometimes knew that they would definitely find a snake, no matter which they picked. Their stress levels were measured through sweating and pupil dilation.

The experiment showed that people were the most stressed when there was a 50 per cent chance of finding a snake and the least stressed when there was no chance, or a 100 per cent chance of receiving a shock.

“Our experiment allows us to draw conclusions about the effect of uncertainty on stress,” said lead author Archy de Berker of UCL.

“It turns out that it’s much worse not knowing you are going to get a shock than knowing you definitely will or won't.

“We saw exactly the same effects in our physiological measures – people sweat more and their pupils get bigger when they are more uncertain.”

Stress in the modern world is often seen as a negative and counterproductive response, but the study also found a potential benefit.

People whose stress responses spiked the most at periods of greatest uncertainty were better at judging whether or not individual rocks would have snakes under them.

“From an evolutionary perspective, our finding that stress responses are tuned to environmental uncertainty suggests that it may have offered some survival benefit,” added senior author Dr Sven Bestmann, also from UCL’s Institute of Neurology.

“Appropriate stress responses might be useful for learning about uncertain, dangerous things in the environment. Modern life comes with many potential sources of uncertainty and stress, but it has also introduced ways of addressing them.

“For example, taxi apps that show where a car is can offer peace of mind by reducing the uncertainty about when it will arrive.

“Real-time information boards at bus stops and train platforms perform a similar role, although this can be undermined by unspecified delays which cause stress for passengers and staff alike.”